is not reflected in the documentation (i.e. it could be that it's
not fully supported), I would suggest to consider two options:

contact tech. support to be sure that your DB will be in the
right (i.e. supportable) state in case you decide to simplify
the process and do column(s) modification. I believe that internally
everything would be done in a right way

or

b) you can stick with dropping and recreating all affected objects
mviews/mview logs -- sure those have to be refreshed then using
'C'omplete method.

There is one major drawback of mviews maintenance, one have to
deal with the fact that due to complexity of the mview object in
most cases when any modification is required the mview has to be
dropped and recreated. It's really painful in case nested mviews
are used and there is a lot of aggregated data, which, unfortunately,
has to be refreshed. In some cases 'ON PREBUILT' mviews can be
considered as an option.